A series of amine-cured epoxy resin systems and the blends of these containing up to 50% polyethersulphone (PES) were evaluated in terms of curing behavior, microstructure, and mechanical and dynamic mechanical thermal properties. The epoxy networks were based on a 2 : 1 mixture of triglycidyl-p-aminophenol and the polyglycidylether of a phenol formaldehyde novolac, and a curing agent of 3,3’-diaminodiphenylsulphone (DDS). The curing mechanism and the rate of curing were affected by the DDS and PES contents. Reaction between the epoxy resin and hydroxyl groups attached to PES was indicated in epoxy-rich formulations. Phase separation, evident in all the blends, was affected by the curing rate. Various blend morphologies were observed depending on the epoxy network/thermoplastic composition. Blending improved ductility and toughness-related properties. particularly at 20-40% PES contents, which corresponded to spinodal/co-continuous morphologies. Changes in glass transition behavior were attributed to possible variations in intermolecular free volume.